1. Field of the Invention
The invention relates to a vehicle steering control system that steers steered wheels on the basis of an operation of a steering member.
2. Discussion of Background
In recent years, there has been proposed a vehicle steering control system equipped with a so-called steer-by-wire system in which a steering member, such as a steering wheel, and steered wheels are not mechanically coupled to each other and part of a steering transmission system is formed of an electrical path. Such a vehicle steering control system includes a steering mechanism that is connected to a steering member and a steered mechanism that actually steers tires with the use of a steered system motor used to steer the steered wheels. The steered mechanism executes drive control on the steered system motor on the basis of a steering angle of the steering member, which is detected by a steering angle sensor.
The steering mechanism includes a reaction motor coupled to a steering shaft, simulates reaction force that is transmitted from a road surface, or the like, to the steered wheels with the use of the reaction motor, and applies the simulated reaction force to the steering member as steering reaction force (see Japanese Patent Application Publication No. 2006-182058 (JP 2006-182058 A)). As methods of controlling the reaction motor, there are a method (A) in which a motor current that is supplied to the reaction motor is directly controlled and a method (B) in which a steering torque that is applied to the steering member is detected with the use of a steering torque sensor and feedback control is executed such that the steering torque approaches a target torque. The method (B) is more accurate than the method (A), and is generally used. A steering torque sensor for an electric power steering is used as the steering torque sensor.
In the steer-by-wire system, a feel that the steering member is rotated to the steering end is also created by the reaction motor. Therefore, large steering torque is applied to the steering member. However, there is a limit on a detection range of the steering torque sensor. Therefore, in order to broaden the detection range of the steering torque sensor, a torsion bar having a large spring constant may be employed as a torsion bar that is a component of the steering torque sensor. However, this may reduce the detection resolution of the sensor within a range in which a steering torque is small.